The phosgenation of aliphatic or aromatic amines for preparing isocyanates can be carried out particularly advantageously in the gas phase. Such processes have in principle been known for a long time in the prior art and have now become established in industry (EP-B 289 840).
A frequent problem is that, for whatever reason, these isocyanates are colored or undesirable secondary reactions occur in subsequent modification steps, for example prepolymerization, biuret formation or trimerization, and ultimately have an adverse effect on the color of the polyisocyanates obtained. The causes and sources of such discoloration are manifold since not all impurities present in the feed streams inevitably lead ultimately to undesirable discoloration in the direct process products or their downstream products.